The technology of drilling gas and oil wells has advanced significantly in recent years. Part of this advancement involves new and improved techniques for drilling non-vertical (i.e. horizontal and other directional) wells. One advantage of horizontal and other directional drilling is that it enables a greater portion of the well bore to be exposed to gas or oil-producing strata, which tend to be disposed more horizontally than vertically. This enables more gas or oil to be produced from the directional well, than from a similar vertical well.
When drilling non-vertical well bores, it is common practice to use downhole sensors to measure the orientation of the well bore. The well orientation information gathered during drilling must be transmitted to the surface. Conventional downhole sensors used to measure well orientation include a three-axis accelerometer used to measure roll and inclination of the well bore, and a three-axis magnetometer (which functions as an electronic compass) to measure the well bore azimuth. Information on the well bore has been transmitted to the surface of the earth using a wireline, a measurement while drilling (MWD) mud pulser, or an electric dipole.
The conventional transmission methods and devices have certain disadvantages. Wireline systems, which use a coaxial high strength cable to connect the downhole sensors to the surface, require the use of a wireline truck. Wireline trucks are expensive, both to buy and operate. Also, the wireline must be cut and reconnected to enable the insertion of drill pipe at the surface as the well is drilled down.
MWD methods require changing the downhole fluid dynamics to propagate pressure pulses to the surface. The pressure pulses are used to encode the downhole information. MWD systems are expensive to buy and operate, and do not work well in some formations which the circulation is lost or poor.
The electric dipole transmission method creates a downhole dipole by electrically isolating a portion of the drill pipe and impressing a voltage across it. This method is relatively simple and inexpensive. However, the technique does not work when there is a moderately conducting formation above the dipole, which shorts the dipole signal. Also, this technique cannot be used inside casing, because casing shorts out the signal.
Magnetic dipole antenna transmission has been proposed to eliminate the above shortcomings but has yet to be perfected for practical usage.
Thus, there is a need or desire for a technique for transmitting downhole data to the surface via a magnetic dipole antenna which is relatively simple and inexpensive, provides strong signal and which can be used in a wide variety of environments.